Amyotrophic Lateral Sclerosis (ALS) is a devastating and rapidly fatal disease with currently only one available, FDA-approved, modestly effective treatment. There is therefore an urgent need for new therapies. With the development of the first genetically based mouse model of ALS in 1994, the field of preclinical testing was energized, but there have been a number of unforeseen complexities along the way.
This document is designed to 1) summarize the current best practices and recommendations available for designing and conducting preclinical studies using currently available SOD1-based mouse models of ALS, and 2) summarize the current best practices and most current information regarding breeding and maintaining SOD1 mutant mouse colonies.
For the purposes of the materials covered in this document, the recommendations to follow focus specifically on preclinical studies, meaning those experimental studies whose primary goal is to develop a therapy for human use. While we believe the following breeding and design recommendations may also benefit general proof-of-concept studies, designed to examine fundamental mechanisms and elucidate new biological targets of ALS, this is not the primary purpose of these materials.
Overview of Various SOD1 Animal Models
As first reported in (Rosen et al., 1993) mutations in the Cu/Zn Superoxide Dismutase 1 gene (SOD1) account for ~20% of Familial ALS (FALS) cases, corresponding to 2-3% of all ALS cases. Transgenic mutant SOD1 mice are the only ALS mouse models currently available that exhibit all of the histopathological hallmarks observed clinically in sporadic and familial ALS.
SOD1 is a ubiquitous, mostly cytosolic, 153 amino acid protein that catalyzes the dismutation of superoxide anion radicals leading to the formation of hydrogen peroxide. The enzyme functions as a homodimer, in which each monomer binds one zinc and one copper atom. Copper binding is thought to be important for catalytic activity while zinc binding is believed to be critical for structural stability.
Over 146 mutations scattered throughout SOD1 have been identified in FALS patients, the majority of these being point mutations of highly conserved amino acids (Cleveland and Rothstein 2001). A continuously updated list of human mutations can be found on the ALSOD online database, alsod.iop.kcl.ac.uk. Intriguingly, all mutations, with the exception of D90A, seem to be inherited in an autosomal dominant manner.
Because there is no obvious mutational hotspot and no clear correlation between the level of enzymatic activity of the mutant SOD1 protein and the observed disease phenotype or clinical progression (refer to Table 1), SOD1 is thought to act primarily via a toxic gain of function in ALS (Pasinelli and Brown 2004, Bruijn et al., 2004), although loss of function may also contribute to disease pathophysiology (Fischer et al., 2007). It is generally thought that the different mutant SOD1 proteins are likely to cause ALS by a similar mechanism. Several hypotheses for SOD1 mutant mediated neuronal loss have been advanced including excitotoxicity, oxidative damage, impaired energy metabolism, inflammation, and insufficient growth factor signaling.
Several transgenic mouse models have been generated that model mutations found in FALS patients (see Table 1 below for comparisons of key characteristics), including the G93A (Gurney et al., 1994), G37R (Wong et al., 1995), G85R (Bruijn et al., 1997), G127X (Jonsson et al., 2004), D90A (Jonsson et al., 2006b), and H46R mutations (Sasaki et al., 2007). In all of these mouse models, massive death of motor neurons 2 Working with ALS Mice in the ventral horn of the spinal cord and loss of myelinated axons in ventral motor roots ultimately leads to paralysis and muscle atrophy. A limited number of other neuronal populations have also been shown to be affected in various SOD1 mutant mouse models, including upper corticospinal motor neurons in G93A mice (unpublished data presented by P. H. Ozdinler in Istanbul, Turkey, July 2009), sensory neurons in dorsal root ganglia in G85R (Bruijn et al., 1997), and neurons of brainstem cranial nuclei in G37R mice (Wong et al., 1995).
All of these mouse models have been reported to exhibit the same histopathological hallmarks associated with ALS in humans: progressive accumulation of detergent–resistant aggregates containing SOD1 and ubiquitin and aberrant neurofilament accumulations in degenerating motor neurons. In addition to neuronal degeneration, reactive astroglia and microglia have also been detected in diseased tissue in the mice, similar to that observed in humans.
Despite these histopathological similarities, the timing of onset and rate of disease progression differ (often dramatically) among the various SOD1 transgenic mouse models. To date, researchers have not be able to account for these differences in onset or progression by looking at particular characteristics of the mutant protein, as disease onset and progression do not appear to correlate with the presence or absence of enzyme activity, or with the stability of the various mutant SOD1 proteins (refer to Table 1 below for summary comparison). However disease progression, but not disease onset, may correlate with aggregation propensity (Wang et al., 2008).
Melanie Leitner, PhD, is Chief Scientific Officer and Sheila Menzies, PhD, is Scientific Program Officer at Prize4Life, and Cathleen Lutz, PhD, is Director of the Mouse Repository and the Rare and Orphan Disease Center at The Jackson Laboratory.